perl.h: explicitly declare "environ" in more places

perl.h: explicitly declare "environ" in more places

Some systems declare environ in some situations; others never do. (BSD
systems seem to fall into the latter category, for example.)

Previously, we declared environ in the absence of the DONT_DECLARE_STD cpp
symbol, but that symbol has now been removed (on the grounds that redeclaring
system functions is typically unhelpful). But it probably shouldn't have been
used to avoid declaring environ, because very few systems actually provide
declarations for that variable.

This change adds the environ declaration almost everywhere, on the grounds
that even where the system headers do declare it, an additional declaration
won't in fact hurt.